I am making a hood/reflector for my HPS and right now it seems that aluminum is really expensive, so I've gone with steel. I was wondering, will the steel by itself work well as a reflector or should I paint it white?
Use some engine paint white of course. Seems like steel will real absorb the heat you should check out some large electrical contractors for some used hoods for warehouse lighting.
Why do you want to paint it? To prevent rust? Bucause it has millscale (it's black) on it?
You can paint it white with some paint that can handle heat. I'vepainted them with flat white alkyd housepaint. They yellowed a bit but the paint held.
If you don't think they'll rust too badly and if you have an angle grinder, you could grind them to bare metal and then maybe put a bunch of "swirl marks" all over it with the grinder to prevent hotspots?
Try whatever. I'd imagine that the worst thing that could happen would be that whatever paint you used didn't adhere and flaked-off, landed on the bulb and somehow caught fire. Pretty bad "worst case scenario".
As was mentioned, engine enamel might be a good choice.